1. Field of Invention
The present invention relates to weld gun arm castings useful in robotic welding.
2. Background
Robotic welding is widely used in modem automotive assembly lines for joining a variety of different parts. In a typical robotic welder, heat for welding is generated by a resistance electrode mounted in a weld gun arm. Since electricity is supplied to the electrode directly through the weld gun arm, the weld gun arm must be designed to carry a high electrical load. In addition, it must be strong and shock resistant. In addition, it must exhibit these properties over a long useful life, as most weld gun arms are designed for making as many as 1 million or more welds.
Accordingly, current industrial specifications call for the best weld gun arms in use today, known as "Class III RWMA" weld gun castings ("RWMA refers to the Resistance Welding Manufacturers' Association), to be made from materials having a minimum Rockwell Hardness of 90B and a minimum electrical conductivity of 45% IACS.
Because of these demanding requirements, and because of their complex shape, most weld gun arms used in the North America automotive industry are made from cast, precipitation hardened copper alloys of the following composition:
Alloy Be wt. % Co wt. % Ni wt. % Balance Class III .35-.80 none 1.80-2.0 Cu (GM/Ford) Class III .35-.50 none 1.30-1.60 Cu (Daim/Crys)
See, in particular, Harkness et al., Beryllium-Copper and Other Beryllium-Containing Alloys, Metals Handbook, Vol. 2, 10th Edition, .COPYRGT.1993 ASM International, which indicates that alloys having high electrical conductivities as well as excellent strength and hardness can be made by adding small amounts of beryllium and nickel to copper metal and then precipitation hardening the alloy so obtained.
Weld gun arms are supplied industrially by fabricators who design different gun arms for different applications. In normal practice, these fabricators also manufacture their own weld gun arms from molten alloys prepared by them. That is, the fabricators prepare one of the above alloys, usually from pure copper metal, pure nickel and a BeCu master alloy containing 4% Be obtained from a beryllium supplier. The fabricator then casts the alloy into a near net shape article--i.e., a casting whose shape is very close to the final shape of the ultimate product desired. The near net shape casting is then worked such as by drilling, cutting or bending to produce the product weld gun arm casting. Precipitation hardening is usually done after fabrication is complete, although it can be done before final shaping if desired.
A significant problem facing the robotic welding industry is premature weld gun arm failure. Most weld gun arm castings are designed for a predetermined useful life such as 1 million or more welds. Many achieve this goal. However, many crack or break long before their useful lives have expired. This often requires shut down of production lines as well as expensive replacement of parts.
Accordingly, it is an object of the present invention to provide new technology for reliably and consistently making weld gun arm castings which not only exhibit the strength, hardness and electrical conductivity of currently available product but also avoid premature failure.
In addition, it is a further object of the present invention to provide new weld gun castings which exhibit the strength, hardness and electrical conductivity of currently available product and, in addition, superior fatigue resistance as compared to currently available product.